1. Field of the Invention
The disclosures herein generally relate to an image forming apparatus, especially an image forming apparatus that has a function to control temperature at a specific portion in the image forming apparatus to a predetermined temperature.
2. Description of the Related Art
FIG. 11 is a general configuration diagram of a conventional image forming apparatus. The image forming apparatus 1 is a monochrome printer. In FIG. 11, the image forming apparatus 1 includes a photoreceptor 201; an image-creating-process unit 211 including a charging device 202 and a developing device 204 that are disposed along the external circumference of the photoreceptor 201 and configure a unit with the photoreceptor 201; an exposure device 203; a transfer device 205; a cleaning device 206; a sheet feeder 207; a fixing device 209; and an electric unit 210. The image-creating-process unit 211 can be attached to or detached from a main body 212 of the image forming apparatus 1 as a whole.
The charging device 202 charges the surface of the photoreceptor 201, which is exposed by the exposure device 203 with colors of image data to form a latent image. The developing device 204 develops the latent image formed on the surface of the photoreceptor 201 by toner to form a toner image, which is transferred from the surface of the photoreceptor 201 to a sheet supplied from the sheet feeder 207 by the transfer device 205. The cleaning device 206 cleans the surface of the photoreceptor 201 after the toner image has been transferred. The fixing device 209 is disposed at a downstream position relative to the transfer device 205.
Sheets which are held in a sheet feeding tray of the sheet feeder 207 are conveyed to a resist roller 208 by the sheet conveying section in response to a printing request. Here, a sheet is conveyed from the resist roller 208 to the transfer device 205 to transfer the toner image at a proper timing. After the toner image has been transferred on the sheet by the transfer device 205, the sheet is conveyed to the fixing device 209, where the toner image is fixed by heat and pressure.
The charging device 202, the developing device 204, and the transfer device 205 are connected with the electric unit 210 to have a predetermined bias applied. The electric unit 210 includes an AC power source (constant voltage), a first DC power source (constant voltage), and a second DC power source (constant current) inside of the electric unit 210. With these power sources, the developing device 204 has a DC voltage applied by constant-voltage control, the charging device 202 is applied with an AC voltage by constant-voltage control, and the transfer device 205 has a DC voltage applied by constant-current control.
In such an electrophotographic image forming apparatus 1, image forming elements in the image forming apparatus 1 tend to be influenced by environmental changes. Especially, characteristics of the elements are change by temperature and humidity, which should be taken care of.
As for temperature, it is known that a temperature rise occurs at various portions in the image forming apparatus 1 such as the exposure (writing) device 203, the fixing device 209, the developing device 204, and a driving motor for driving the photoreceptor (image bearing member) 201 or the like to rotate. For example, frictional heat may cause a temperature rise at the developing device 204, which is generated between the developer and a member for stirring and conveying the developer when they are rubbed together by an operation of the member, or within the developer for the same reason.
Also, frictional heat is generated by friction between the developer on a developer bearing member and a developer restricting member that restricts the layer thickness of the developer before the developer is conveyed to a development area, by friction within the developer when restricted by the developer restricting member, which may cause a temperature rise in the developing device 204.
If temperature rises in the developing device 204, toner charge is reduced while toner adherence increases, which make it difficult to obtain a predetermined image density. The temperature rise also makes the toner melt to condense. The condensed toner may adhere to the developer restricting member, the developer bearing member, the image bearing member, etc., which induces a risk that an abnormal image with stripes is generated. Especially, in recent years, low-melting-temperature toner has been used to decrease fixing energy, with which abnormal images are likely to be generated due to the condensed toner.
As for humidity, the air composition in the image forming apparatus 1, especially humidity, may destabilize image quality because an image is created by the photoreceptor 201 with charged fine particles such as toner, carriers, and the like, whose charged state is affected by humidity in the image forming apparatus 1. For example, the fine particles such as toner, carriers, and the like used in a electrophotographic device are designed to be made of polymeric resin with a charge controlling agent added to stabilize the electrostatic charged state of the particles. However, electric characteristics of polymeric resin cause absorption of moisture in the air to induce changes in electrical resistance, friction coefficient between granular materials, fluidity, and the like even if hydrophobic treatment or the like is applied. Consequently, the amount of toner charge in the developing device 204 is reduced, which results in image quality changes such as an increased density.
In the charging device 202 using electrical discharge phenomenon, when an electrical discharge occurs, a nitric acid compound is generated, then, is combined with moisture in the air to generate ionized materials such as nitric acid and nitrate, which adhere to the surface of the photoreceptor 201. With the adhesion, the surface of the photoreceptor 201 deteriorates fast, which causes an abnormal abrasion of the photoreceptor 201. Also, ionized materials make the surface conductive, which generates fuzz on an electrostatic latent image, or an image deletion.
To cope with these problems primarily, an air-conditioning section may be provided. As a general air-conditioning section, a vapor-compression-type refrigerator is shown in FIGS. 12A-12C with its configuration and principles of operation. FIGS. 12A-12C are schematic views illustrating the configuration and principles of operation of a vapor-compression-type refrigerator as a general air-conditioning section. In FIG. 12A, the vapor-compression-type refrigerator is configured with a compressor 101 for compressing coolant, a first heat exchanger (condenser) 102 for heat exchange between the coolant and the air, a second heat exchanger (evaporator) 103, an expansion valve 104 for decompressing the coolant, and a four-way valve 105 for switching passages of the coolant. The vapor-compression-type refrigerator can heat or cool the air by repeating the following cycle.
1. Compression
Low-pressure, low-temperature coolant vapor is compressed by the compressor 101, to generated high-pressure, high-temperature coolant vapor.
2. Condensation
The high-pressure, high-temperature coolant vapor generated by the compressor 101 is fed to the first heat exchanger 102 to exchange heat with the air for cooling, to become liquid coolant (the air is heated).
3. Expansion
The high-pressure liquid coolant liquefied by the first heat exchanger 102 is decompressed by the expansion valve 104.
4. Evaporation
The liquid coolant decompressed by the expansion valve 104 is evaporated by the second heat exchanger 103 to absorb heat from the air (the air is cooled).
The above process is explained with a T-s diagram (temperature entropy diagram) on temperature T and entropy S in FIG. 12C. The compressor 101 compresses dry saturated vapor A of the coolant up to a pressure above the saturated vapor pressure corresponding to a predetermined temperature in the first heat exchanger 102 to generate overheated saturated vapor B, which is fed to the first heat exchanger 102. The overheated saturated vapor B fed to the first heat exchanger 102 discharges heat Q1 by heat exchange with the air to be liquefied to become saturated liquid C. The saturated liquid C is fed to the expansion valve 104 to undergo isenthalpic expansion in the expansion valve 104 and to become wet vapor D, which is fed to the second heat exchanger 103. The wet vapor D fed to the second heat exchanger 103 absorbs heat Q2 from the air, to be evaporated to become dry saturated vapor A again.
FIG. 12B shows a state of the four-way valve 105 switched from a state shown in FIG. 12A. Namely, the passage of the coolant shown in FIG. 12A that goes through the compressor 101, the first heat exchanger 102, the expansion valve 104, and the second heat exchanger 103 is changed by the four-way valve 105 to the passage of the coolant shown in FIG. 12B that goes through the compressor 101, the second heat exchanger 103, the expansion valve 104, and the first heat exchanger 102 so that the first heat exchanger 101 and the second heat exchanger 103 function both ways as a condenser and an evaporator, or an evaporator and a condenser, respectively.
If an image forming apparatus as a whole is to be cooled by such an air-conditioning device, the capacity to be controlled becomes too large to keep modest cost, size, noise, and power consumption.
Thereupon, an air conditioning technology is disclosed that partially cools down only an image creation portion including a photoreceptor, a developing unit and the like, for example, in Japanese Laid-open Patent Publication No. 2003-122208 (Patent document 1) and Japanese Patent No. 3924484 (Patent document 2).
In Patent document 1, a technology that aims to efficiently remove a material harmful to image forming from the surroundings of a photoreceptor is disclosed as follows. An image forming apparatus includes a main body case, an image forming unit having a photoreceptor housed in an image forming case, and an air-conditioning section. The image forming unit has an opening formed for image transfer where a part of the photoreceptor is exposed. The opening is the only opening through which a material harmful to image forming may flow into the image forming unit when attached to the main body case. The air-conditioning section removes a material harmful to image forming which may flow into the image forming unit from the outside. The image forming apparatus also includes a passage that starts from the outside of the image forming unit, goes through the inside of the image forming unit, and goes out the outside of the image forming unit. At the entrance of passage, the air-conditioning section is disposed. The image forming apparatus also includes a circulation passage along which air inside of the image forming unit is discharged out of the image forming unit and again introduced into the image forming unit. The air-conditioning section is also disposed in the middle of the circulation passage.
In Patent document 2, a technology is disclosed that aims to maintain satisfactory performance of scraping residual toner on a photosensitive body with a cleaning blade in an electrophotographic image forming apparatus even if temperature and humidity are changed in an environment where the electrophotographic image forming apparatus is installed. The image forming apparatus includes: a temperature adjusting device for heating or cooling air to supply heated or cooled air; an air-conditioning section having a function of adjusting temperature of the cleaning blade; a temperature sensor for measuring the temperature of the cleaning blade; and a temperature control section which drives the air-conditioning section in response to a detected temperature by the temperature sensor and a reference temperature set in advance so that the cleaning blade scrapes off residual toner appropriately at a controlled temperature. At least the photosensitive body, a developing unit and a cleaning unit are held in a well-closed case of an image creation module where image forming takes place for a color. Multiple image creation modules are provided for different colors. Temperatures in the image creation modules are adjusted by the air-conditioning section to keep the reference value of temperature set for each of the image creation modules.
Also disclosed in Patent document 2 is an image forming apparatus includes a humidity adjusting section for adjusting humidity used instead of or along with the temperature adjusting section, which refers to a reference value of humidity instead of or along with the reference value of temperature.
In an image forming apparatus, when controlling temperature with an air-conditioning device, it is difficult to achieve fine control of temperature only with the air-conditioning device. In addition, if there are multiple portions in the image forming apparatus whose temperatures need to be controlled at different target temperatures, it is difficult to achieve the target temperatures at the temperature-controlled portions, respectively, because a cooled or heated wind at substantially the same temperature is sent to all the temperature-controlled portions.
On the other hand, it may be possible to individually control the amount of flow of the air sent to each temperature-controlled portion for heat exchange so that the temperature of each temperature-controlled portion can be controlled. In this case, the temperature and amount of flow of the air sent from the air-conditioning device are not independent, hence it requires appropriate control of a compressor of the air-conditioning device, output of a fan (the number of rotations) of a heat exchanger, the amount of flow sent to each temperature-controlled portion, and the like. However, in a situation where the amounts of heat at portions in the image forming apparatus are changing considerably, for example, soon after a starting-up or a change of operation mode, it is difficult to control the temperatures of the temperature-controlled portions appropriately.